Structural and spectroscopic investigations of aminocyclopropenium cations, [C3(NR2)3]+(NR2= NMe2, NEt2, NC5H10 or NC4H8O) and [C3(NPr i2)2Cl]+

Abstract

The structure of tris(piperidino)cyclopropenium perchlorate, determined by X-ray crystallography, reveals the cation [C₃(NC₅H₁₀)₃]⁺ in which short C—C distances of 1.371(4), 1.382(4) and 1.391(4)Å, and C—N distances averaging 1.333(4)Å point to electron delocalisation over the C₃N₃ core. The geometry of the related bis-1,2-(diisopropylamino)-3-chlorocyclopropenium ion, [C₃(NPrⁱ₂)₂Cl]⁺, is marked by two C—C bonds of 1.329(4) and 1.332(4)Å, with a longer bond of 1.427(5)Å between the ring carbons bearing the NPrⁱ₂ substituents. The C—Cl bond length is 1.723(3)Å while the exocyclic C—N bonds measure 1.272(4) and 1.299(4)Å. ¹H and ¹³C NMR data are used to characterise the title compounds, to detect the intermediates C₃(NC₅H₁₀)₂Cl₂ and [C₃(NC₅H₁₀)₂Cl]⁺ in the reaction which produces [C₃(NC₅H₁₀)₃]⁺ from C₃Cl₄, and to measure the activation energy for restricted rotation of the C—N bond, ΔG^‡₂₉₈= 75 kJ mol^–1, in the case of [C₃(NPrⁱ₂)₂Cl]⁺. A ‘ring-breathing’ mode forms an intense feature near 1950 cm^–1 in Raman spectra of aminocyclopropenyl ions; their other IR and Raman bands are assigned, including a detailed vibrational analysis of the [C₃(NMe₂)₃]⁺ ion.